This invention relates to heavy duty pneumatic tires for use in heavy vehicles such as trucks and buses, having tread patterns incorporating anti-skid grooves of a shape to minimize stone pick-up and stone retention without reducing wet-grip performance of tires.
Pneumatic tires are conventionally provided with a tread surface having an anti-skid pattern defined by circumferential grooves or, and transverse grooves which are molded in the surface of the tread during manufacture.
In general, a pneumatic tire has a tread patterns of the lug, rib, rib-lug, block, lug-block or rib-block pattern type which is selected in accordance with performances required for the tire depending on the use of the tire, reasonal factors, road conditions, etc. especially a pattern of tread grooves for heavy duty tires for use in heavy vehicles such as trucks and buses are mainly used with a tread pattern of the rib, rib-lug or rib-block type pattern with respect to the strong requirement for wet grip performance and long wear life tread performance.
However, such a tread pattern having circumferentially extending longitudinal grooves has the following drawbacks when compared with the other tread patterns such as a lug-type pattern.
That is, stone pick-up is apt to occur in the circumferential groove. If this occurs and the tire is driven, it is difficult to discharge stones from the groove due to the strong restraining force of the groove. As a result, the stones strike the bare rubber between the bottom of the circumferential grooves and the reinforcement for the tread portion on the belt for every rotation of the tire and tend to cause the breakage of the bare rubber and hence damage the belt layers or carcass plies inside the tread rubber. Particullarly, when the belt is comprised of metal cords, rust is induced by penetrating water from the damaged portion of the bare rubber into the belt or the carcass plies and as a result, separation failure of metal cords from the rubber is caused. Finally there is the fatal possibility of damaging the durable life of the tire.
As a countermeasure against such stone pick-up, it has hitherto been proposed to arrange a groove with a step ( -shaped groove) continuously extending toward the circumferential direction of the tire on the groove walls of the longitudinal groove and to arrange a protoruded stripe continuously extending toward the circumferential direction of the tire on the groove bottom. In the former case, the stone pick-up is somewhat protected in the early stage of the tire use by the wider groove and the narrower groove, However when the wider grooves disappeared due to tread wear, only the narrower groove remains, and as a result, there is a problem to reduce the wet grip performance in the worn tire saving only the narrower groove. In the latter case, the bare rubber on the groove bottom is somewhat protected, but it can not practically and satisfactorily be expected to facilitate the discharge of picked up stones.